commonly used conventional controllers, like PID controllers (which provide some combination of a proportional, integral and differential output), are typically 1 port controllers. This means that the controller uses a single control law which is applied to both the reference (set point) processing and the disturbance/load processing simultaneously. (The disturbance/load signal consists of the plant output signal as well as any disturbance.) If the reference and disturbance/load time profiles have different properties, as is almost always the case, the 1-port controller cannot be optimal. Because the 1 port controller mixes both the reference and the disturbance/load signals into one signal, the individual contributions from each cannot be determined.
The optimal solution can be found, not by trying to reconcile the two, but by treating each one individually. To accomplish this, a 2-port configuration is required, in which each signal is processed independent of the other, without any mutual interaction.
The invention relates to nonlinear, time-variable digital controllers and a method for their synthesis. Unlike conventional control systems, in which the processing of the reference load and the disturbance changes is carried out in the same unit, the cycling controllers herein keep them totally separated. This benefits the design because any control task can be treated as a pure tracking problem in which one unit is responsible for planning and synthesizing the controlled system's trajectory to move it into a desired state, and another unit takes care of possible deviations from the trajectory due to disturbances or load changes. Since the two units are independent, their characteristics can be chosen arbitrarily. This new functionality enables the design of control systems that surpass conventional systems in the quality of control.
To exemplify its use, the invention has been applied to developing a fixed-rate controller for electric utilities. In the fixed-rate mode the controller herein provides rate-optimal control of the controlled plant. The rate-optimal control produces the fastest plant response that never exceeds the user-specified rate. On the other hand, being the fastest implies that most of the time, the controller drives the plant at or near the maximum user-specified rate. As a result, in the fixed-rate mode the controller herein offers the maximum dynamic responsiveness of the plant without excessively stressing its components for any arbitrary set point or load change time profile.
In today's utility industry, the use of ever larger plants as spinning reserve capacity has made rapid response to large load swings a necessity. The issues of generation uncertainty caused by cogeneration and independent power producers and dwindling reserve capacity have magnified the load response problem. Larger daily load swings have also necessitated that plants be stable but very responsive. The industry also requires control solutions that not only can respond rapidly to load swings, but help minimize process upsets and oscillations of the process. Oscillations and large process upsets degrade the performance and reduce the life expectancy of a plant. Overall plant performance and the ability to optimize and feed back performance status to both the control system and the dispatcher will play an important role in the application of control technology in the power plant.
The power generation industry could realize significant benefits from this development which could potentially revolutionize the application of control to power plants to realize the following benefits:
Increased plant life and reduced component failures through true fixed rates of change based on critical stress levels. PA1 Improved system frequency response through rapid control mode switching for fixed-duration, fixed-rate response. PA1 Optimized response to dispatched load changes through response-based mode feedback to the dispatch center. PA1 Improved heat rate through improved process stability and performance-based mode of control. PA1 Improved process stability through fixed-rate control rather than through plant response feedback. PA1 Minimized start-up cost with the start-up and shutdown mode of control. PA1 More avenues for subloop improvements become available with the application of an advanced control technology as the foundation for plantwide control.
There is thus a trend toward developing larger manufacturing and production units because they provide higher efficiency. Controlling such units gives rise to problems not encountered with smaller systems. The energy that must be absorbed or released by the controlled system as a result of control actions is often so large that thermal, kinetic, torque, vibration and other stresses can easily damage the unit if the system state change is too rapid.
This issue is of prime importance to electric utilities. The current state-of-the-art hardware (for power plant control) is adequate to meet the needs fo the future. Only emphasis on the development of new control strategies and approaches is necessary to achieve improved performance.